The technical part of the optician's profession involves mounting a pair of ophthalmic lenses on a spectacle frame selected by a wearer.
This mounting is divided up into three main operations:                acquisition of the contours of the surrounds of the selected spectacle frame,        centering of each lens, which consists in appropriately positioning and orienting the acquired contours relative to the ophthalmic lenses so that, once assembled on the spectacle frame, each lens is suitably centered on the pupil of the corresponding eye of the wearer, then        machining of each lens, which consists in cutting them along these contours.        
In the context of the present invention, interest is more particularly focused on the spectacle frames with surrounds, that is to say on full-rim and half-rim (“arched”) spectacle frames.
For these frames, the optician's physical objective is to cut the ophthalmic lenses so that they can be mechanically and esthetically adapted to the shapes of the surrounds, while ensuring that these lenses best fulfill the optical functions for which they were designed.
The machining operation comprises in particular, in the case of full-rim frames, a beveling step used to form on the edge of the lens a fitting rib, commonly called bevel, capable of being fitted into a groove which runs along the internal face of the corresponding surround of the frame.
On the other hand, in the case of half-rim frames, the machining operation includes a grooving step with which to form a fitting groove on the edge of the lens. When mounting the lens in the surround, this fitting groove is engaged on a rib that runs along the internal face of the corresponding half-rim (or “arch”) of the frame. The ophthalmic lens is then held bearing against this arch with the help of a thread, generally made of nylon (possibly of metal), which is engaged in the fitting groove and the ends of which are connected to the ends of the arch.
Whatever the type of spectacle frame selected, the acquisition and machining operations must be carried out with care so that the lens can be fitted perfectly into its surround, without force and “at the first attempt”, that is to say without requiring any remachining or any bending of the nylon thread.
To acquire the shape of the surround, a contour reading appliance is generally used that comprises a feeler which slides directly over the surround or over a presentation lens characteristic of the shape of this surround.
However, at the end of this feeling operation, reading errors inherent to the operation of the reading appliance are observed. At the end of the machining operation, machining errors that are also inherent to the operation of the trimming appliance are also observed.
By reducing these errors, the applicant has observed that some ophthalmic lenses remain difficult to mount in their surrounds. It is then necessary, to dispel any risk of the lens slipping out of its surround, to remachine the lens and/or to modify the length of the nylon thread, which is tedious to do.